Materials are staying the same as another object even if changed
You are not setting the material in the correct place. The tab you have set the material is for the material override, which sets all object on the render layer to the same material.
See https://docs.blender.org/manual/en/latest/render/post_process/layers.html

Instead you should clear the material from there and set it on the materials tab (the one with the ball symbol). See https://docs.blender.org/manual/en/latest/editors/properties_editor.html#tabs